Climber usually wear chalk bags—little pouches full of chalk that they belt to their waists and that dangle around as they climb. But when you're bouldering, you're only climbing say 20 feet, you don't need a whole bag of unwieldy chalk—so you can just smear some on the side of your pants. It's not that wasteful, and leaves you far more unencumbered. Bouldering is like swimming—elegant, all about core—you don't really want stuff hanging off you.